This invention relates to a fluorescent display device which includes a semiconductor element for driving arranged on an extension of a substrate constituting a part of a hermetic envelope which is formed at the substrate so as to be positioned outside the envelope, and more particularly to a fluorescent display device which permits the semiconductor element to be removed even after it is mounted on the fluorescent display device.
A fluorescent display device is requested to provide a display of high density. Concurrently, the fluorescent display device is requested to simplify lead wires for feeding the fluorescent display device with an electric power therethrough to facilitate lead-in and lead-out of the lead wires. For this purpose, the fluorescent display device is often constructed in such a manner that a semiconductor element for driving is mounted on an outside of an envelope or an extension of a substrate constituting a part of the envelope which is formed at the substrate so as to be positioned outside the envelope.
A way in which the semiconductor element is mounted directly on the substrate to connect a wiring on the substrate to a terminal of the semiconductor element includes a face-down procedure. The face-down procedure is classified into some types.
One of the types of the face-down procedure is to use a conductive adhesive material. More particularly, this is to connect metal bumps of the semiconductor element through a conductive adhesive material to terminals on the substrate and then seal them by means of epoxy resin or the like.
Another type is practiced using an anisotropic conductive sheet, wherein a film having conductive particles uniformly dispersed in an adhesive material is interposedly arranged between bumps of the semiconductor element and terminals on the substrate and subject to a heating treatment under pressure, resulting in connection between both being carried out through the conductive particles.
A further type of the face-down procedure is to utilize direct connection by means of Au bumps. This is to connect Au bumps of the semiconductor element to terminals on the substrate and then seal them by means of epoxy resin or the like.
In addition, other connection ways such as connection by soldering, connection using conductive plated resin balls and the like may be employed for this purpose.
Now, an example of a manner in which the semiconductor element for driving is arranged on the substrate of the fluorescent display device will be described with reference to FIGS. 2 and 3.
A fluorescent display device shown in FIGS. 2 and 3, includes a substrate 1 made of a glass plate or the like, which is provided on an inner surface thereof with an anode conductor 2 for forming a display pattern. For this purpose, the anode conductor 2 is provided thereon with phosphor layers 3. Also, the fluorescent display device includes a wiring conductor 2a electrically connected to the anode conductor and other electrodes during formation of the anode conductor 2. The wiring conductor 2a is covered with an insulating layer 4.
Further, the fluorescent display device includes control electrodes 5 arranged above the substrate 1 and cathodes 6 stretchedly arranged above the substrate 1 and control electrodes 5 through supports 9.
The substrate 1 is integrally joined to side plates 7 and a front cover 8 by means of a sealing medium made of low-melting frit glass, resulting in a hermetic envelope being formed. The joining operation using the sealing medium is carried out at an operating temperature of about 400.degree. C. sufficient to permit residual gas in the envelope to be discharged therefrom.
In the fluorescent display device thus constructed, electrons emitted from the cathodes 6 are selectively impinged on the phosphor layers while be selectively controlled by the control electrodes 5, resulting in providing a desired luminous display.
The substrate 1 is arranged so as to outwardly extend at a part thereof from the hermetic envelope, to thereby provide an extension 1a on which a semiconductor element 12 is supportedly mounted as described below. The wiring conductor 2a is arranged so as to extend onto the support section 1a. The semiconductor element 12 is provided with connections 13 and the insulating layer 4 is formed with an opening in a manner to positionally correspond to each of the connections 13 of the semiconductor element 12, through which opening the wiring conductor 2a is exposed. Mounting of the semiconductor element 12 on the extension 1a of the substrate 1 is carried out in such a manner that electrical connection between the wiring conductor 2a and the semiconductor element 12 is executed by means of conductive adhesive as described above and then sealed by means of resin 14 such as epoxy resin or the like.
Unfortunately, sealing of the semiconductor element 12 using the epoxy resin 14 causes a failure in insulation of the semiconductor element due to deterioration in moisture resistance of the resin and tight adhesion between the resin and the substrate.
Also, use of the conductive adhesive or anisotropic conductive sheet leads to an increase in contact resistance between the metal bumps of the semiconductor element 12 and the wiring conductor 2a on the substrate 1.
Further, direct connection between the semiconductor element and the terminals by means of the Au bumps or using the conductive plated resin balls permits the contact resistance to be somewhat reduced. However, such connection deteriorates reliability in the connection due to a failure in an increase in pressing force of the semiconductor element 12 against the wiring conductor 2a because the pressing force is basically determined depending on shrinkage force of the resin.
Furthermore, the connection by soldering described above fails to accommodate to high density mounting.